1) Field of the Invention
The present invention relates to an observation optical system. To be specific, it relates to an observation optical system used in an image display apparatus that can be held on the head or face of an observer and that can be attached to a cellular phone or a portable intelligent terminal.
2) Description of Related Art
In recent years, development has been energetically made for image display apparatuses, specifically for those to be held on the head or face of individuals for entertaining them with a wide-screen image.
Regarding the conventional observation optical systems applicable to such apparatuses, each of U.S. Pat. No. 4,613,200, International Application No. PCT/US99/26756 (Publication No. WO 00/28369), Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 5-178120, Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 5-077658 etc. proposes an image display optical system that achieves compensation for chromatic aberrations by combining two reflection-type volume hologram elements together.
Also, each of U.S. Pat. No. 4,309,070, U.S. Pat. No. 4,711,512, U.S. Pat. No. 5,966,223, Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 8-113059 etc. proposes an image display optical system that includes a combination of a volume hologram element and a light-transmitting plate.
Also, each of Japanese Patent Application Preliminary Publication (KOKAI) No Hei 6-294943, Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 7-134266, Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 7-218859 etc. proposes an image display optical system that includes a relay optical system and a combiner.
Also, each of Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 2-297516, U.S. Pat. No. 5,880,888 etc. proposes an image display optical system that has a relay and uses a light-transmitting plate.
However, an optical system as recited in U.S. Pat. No. 4,613,200, JP KOKAI No. 5-178120 or JP KOKAI No. 5-077658 becomes thick in the direction of line of sight, which is a problem. In reference to FIG. 1 of U.S. Pat. No. 4,613,200, for instance, this is because the optical system is configured to provide a reflection-type volume hologram element on an entrance surface of a tilting windshield (combiner) of an automobile for the purpose of deflecting incident light, which comes from a direction substantially perpendicular to the line of sight, in a direction along the line of sight.
Also, the optical system disclosed in WO 00/23869 is configured so that, as shown in FIG. 22 of the document, a reflection-type volume hologram element is disposed in the path of rays for image display and deflects the path, and that a transmission-type volume hologram also is incorporated so that chromatic aberrations cancel out. However, such a configuration with a reflection-type hologram element and a transmission-type hologram element requires a large reflecting diffraction angle at the combiner so as to guide the light reflected from the combiner by diffraction in a path that avoids the face of an observer. Consequently, a large power derived from diffraction effect of the reflection-type volume hologram element is needed at the combiner, and thus a problem is raised in that producing such a hologram having high-density interference fringes is difficult.
Also, in an optical system as disclosed in U.S. Pat. No. 4,309,070, U.S. Pat. No. 4,711,512, U.S. Pat. No. 5,966,223, KOKAI No. 8-113059 or the like, a combination of a volume hologram element and a light-transmitting plate forms a path of rays, to achieve thin shape of the optical system. However, no consideration is made for laying out the entire observation optical system along the face of an observer, to cause bulkiness of the entire observation optical system, which is a problem.
Also, in an optical system as disclosed in JP KOKAI No. 6-294943, JP KOKAI No. 7-134266, JP KOKAI No. 7-218859 or the like, a reflecting surface is used only at one portion of the combiner and thus the relay optical system cannot be laid out along the face of an observer, which is a problem.
Also, in an optical system as disclosed in JP KOKAI No. 2-297516, U.S. Pat. No. 5,880,888 or the like, no particular means or configuration is proposed for compensation for chromatic aberrations and decentered aberrations.
Therefore, an object of the present invention is to provide an observation optical system that is so compact and lightweight as to be applicable to a head-mount-type virtual image observation apparatus, provides a bright image with high resolution, and is easily assembled.
An observation optical system according to the present invention comprises an image display element, a relay optical system which relays image information formed on the image display element, and an eyepiece optical system which introduces the image information relayed by the relay optical system into the eye of an observer. The relay optical system includes, at least, a plurality of lenses and a first reflection-type volume hologram element. The first reflection-type volume hologram element is constructed and arranged to have a power and to compensate for chromatic aberrations. The plurality of lenses are constructed and arranged to compensate, at least, for decentered aberrations and chromatic aberrations. The eyepiece optical system has a second reflection-type volume hologram element, which is constructed and arranged to reflect bundles of rays toward the eye of the observer, to exert a power on the bundles of rays, and to compensate for chromatic aberrations. A light-transmitting plate is sandwiched between the first reflection-type volume hologram element and the second reflection-type volume hologram element, is filled with a transparent medium, and is constructed and arranged to lead the bundles of rays, which are reflected at least by the first reflection-type volume hologram element, to the second reflection-type volume hologram element by letting the bundles of rays cause, in the path, total reflection odd-numbered times equal to or greater than three times. The light-transmitting plate allows the relay optical system and the image display element to be arranged along the face to the side head of the observer.
This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.